An accelerometer is one of the major sensors used in navigational systems, particularly inertial navigational systems, and on-board automotive safety control systems. Automotive examples of accelerometer use include various anti-lock braking systems, active suspension systems, and seat belt lock-up systems.
More generally, an accelerometer is a device which measures acceleration and, in particular, an accelerometer measures the force that is exerted when a moving body changes velocity. The moving body possesses inertia, which tends to resist the change in velocity. It is this resistance to a sudden change in velocity that is the origin of the force which is exerted by the moving body, and which is proportional to the acceleration component in the direction of the movement, when the moving body is accelerated.
In a typical accelerometer, a mass is suspended by two springs attached to opposite sides of the mass. The mass is maintained in a neutral position so long as the system is at rest or is in motion at a constant velocity. When the system undergoes a change in velocity in the direction of the springs, axis, and therefore is accelerated in that direction, the spring mounted mass will at first resist the movement because of its inertia. Therefore, this resistance to the movement, or delay, will force one of the springs to be stretched and the second to be compressed. The force acting on each spring is equal, yet opposite, in magnitude and is equal to the product of the weight of the mass and the acceleration of the mass. The acceleration which is determined is the change in velocity experienced by the mass.
Microaccelerometers having resonant microbridges are also known. An example of this type of accelerometer is disclosed in U.S. Patent application Ser. No. 052,026 to Howe et al., entitled, "Resonant Accelerometer." In a microaccelerometer of this type, a proof mass is suspended by at least one pair of resonant microbridges. The pair of resonant microbridges are attached to opposite ends of the proof mass along a common axis. This type of resonant microaccelerometer is attractive for precision measurements because the frequency of a micromechanical resonant structure can be made highly sensitive to physical or chemical signals.
In a microaccelerometer based on resonant microbridges, the acceleration in the plane of the substrate causes differential axial loads on oppositely disposed resonant microbridges, i.e., causes one supporting resonant bridge to be in compression and the other in tension. It is the inertial force of the proof mass which generates the axial load on the resonant microbridges. In turn, the compressive and tensile loads shift the inherent resonant frequencies for each resonant microbridge. The difference between the resonant frequencies of the compressive and tensile members is measured and used to determine the magnitude of the acceleration component in the direction of the common axis shared by the pair of resonant microbridges.
A shortcoming of the above-identified U.S. patent application Ser. No. 052,026 is that the accelerometer is not as durable or rugged as is preferred for automotive applications. In particular, the drive and sense electrodes, which maintain the microbridges at their respective resonant frequency and sense the changes in their resonant frequency when the microbridge is axially loaded, are cantilevered silicon beams which extend over and above the resonant microbridges. Therefore, the cantilevered electrodes are extremely fragile and susceptible to damage. It would be preferable to alleviate the cantilevered electrodes and form the electrodes so that they are integral with the supporting solid silicon substrate of the microaccelerometer. This arrangement would eliminate vibration of the drive and sense electrodes and therefore extend the life of these components and increase the accuracy of the sensor. In addition, this arrangement would reduce the complexity of the fabrication process.
Another shortcoming of the resonant bridge microaccelerometer disclosed in U.S. Ser. No. 052,026 is that the resonant bridges, which suspend the proof mass, are formed from single crystal silicon doped with boron. This material causes the resonant bridges to be under an inherent tensile stress. It would be advantageous to form the resonant bridges in such a manner so as to result in the bridges being optimally stress-free or under minimal compressive stresses.
In addition, for this type of microaccelerometer, it is desirable to limit the displacement of the proof mass in the direction perpendicular to the proof mass. Generally, this type of microaccelerometer has two pairs of resonant bridges, each member of the pair located along a common axis through the proof mass and each pair located along a perpendicular axis with respect to the other pair. Therefore, for convenience of description, the resonant bridges are typically located along an x and y axis through the proof mass, and measure acceleration in those directions. If acceleration is not being measured in the third direction perpendicular to the proof mass, i.e., the z-axis direction, it is desirable to limit the displacement of the proof mass in that z-axis direction, so as to maximize the life of the device and so as to minimize any detrimental effects this z-directional displacement has on the x and y axis measurements. The current resonant microaccelerometer does not provide a means for limiting this z-directional displacement.
Although there are several different types of accelerometers currently available commercially, they are generally typified by the same problems, in addition to those shortcomings particular to the above-described structure. First, an x-direction acceleration signal is often affected by y- or z-direction acceleration and vice versa, or by non-signal motions. Second, a change in temperature may induce stress variation and hence cause inaccurate measurements. Lastly, the current accelerometers are characterized by extremely high cost to produce without the requisite long life durability for highly rugged applications such as in an automotive environment.
Current accelerometers are unable to meet the requirements of reliability, accuracy, ruggedness, and low cost, all characteristics which are required for on-board automotive systems, as well as other applications. Therefore, it is desirable to provide a resonant-bridge microaccelerometer which alleviates the above-mentioned detriments, in particular by providing drive and sense electrodes which are integral with the supporting substrate, resonant bridges which are inherently stress-free or under a minimal compressive stress, and further, means for limiting the z-directional displacement of the proof mass.